1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having reduced flicker.
2. Description of the Prior Art
A thin film transistor display, such as a thin film transistor liquid crystal display (TFT-LCD), utilizes many thin film transistors, in conjunction with other elements such as capacitors and bonding pads, arranged in a matrix as switches for driving liquid crystal molecules to produce brilliant images. The advantages of the TFT-LCD over a conventional CRT monitor include better portability, lower power consumption, and lower radiation. Therefore, the TFT-LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), electronic toys, etc.
Referring to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a prior art TFT-LCD 10. FIG. 2 is an equivalent circuit diagram of the TFT-LCD 10. The TFT-LCD 10 comprises a scanning line control circuit 12, a signal line control circuit 14, and a pixel array 16 having a plurality of pixels connected to scanning lines. For example, a pixel A, a pixel B, and a pixel C are connected to a common scanning line. As shown in FIG. 2, a pixel 20 comprises a liquid crystal cell (LC), connected to a common counter electrode (CE), and a thin film transistor (TFT), which comprises a gate electrode connected to a scanning line G0, a drain electrode connected to a signal line D0, and a source electrode connected to a pixel electrode of the liquid crystal cell. Additionally, the pixel 20 contains a storage capacitor (SC) connected between the liquid crystal cell and a scanning line G1. The storage capacitor is used to reduce the voltage variation of the liquid crystal cell due to current leakage and thus help the liquid crystal cell to store electric charges.
As shown in FIG. 2, the light passing through the pixels varies with the voltage applied to the liquid crystal cell. By changing the voltage to the liquid crystal cell, the amount of light passing through each pixel can be changed and thus the TFT-LCD can display predetermined images. The voltage applied to the liquid crystal cell is the difference between the voltage of the common counter electrode and the voltage of the pixel electrode. When the thin film transistor is turned off, the pixel electrode is on a floating status. If any fluctuations occur in the voltages of electric elements around the pixel electrode, the fluctuations will cause the voltage of the pixel electrode to deviate from its desirable voltage. The deviation of the voltage of the pixel electrode is referred to feed-through voltage (VFD), which is represented by:VFD=[CGS/(CLC+CSC+CGS)]*ΔVG  (1)
where CLC is the capacitance of the liquid crystal cell (LC), CSC is the capacitance of the storage capacitor (SC), CGS is the capacitance between the source electrode and the gate electrode of the thin film transistor, and ΔVG is the amplitude of a pulse voltage applied to the gate electrode.
In general, adjusting the voltage of the common counter electrode can compensate for the feed-through voltage. However, because the resistance and the capacitance of the scanning line round the falling edge of a pulse voltage applied to the gate electrode, a feed-through voltage of a pixel decreases as the distance between the scanning line control circuit and the pixel increases. For example, as shown in FIG. 1, feed-through voltage of the pixel A is larger than that of the pixel B, whose feed-through voltage is larger than that of the pixel C (that is, (VFD)A>(VFD)B>(VFD)C where (VFD)A,(VFD)B, and (VFD)C represent feed-through voltages of the pixels A, B, C, respectively). Accordingly, it is difficult to compensate feed-through voltages for all pixels by adjusting the voltage of the common counter electrode. Therefore, it is hard to provide a TFT-LCD without flicker.
The method disclosed in U.S. Pat. No. 6,028,650 attempts to solve the above-mentioned problem. Referring to FIG. 3. FIG. 3 is a top view of a pixel array 30 of the TFT-LCD 10. The pixel array 30 comprises scanning lines 32 and 32a connected to a scanning line control circuit (DR1), signal lines 34a, 34b, 34c, and pixels A, B, C, which correspond to pixels A, B, C shown in FIG. 1. Pixels A, B, C comprise thin film transistors QA, QB, QC respectively, and their corresponding liquid crystal cells. The gate electrodes of thin film transistors QA, QB, QC are connected to the scanning line 32. The drain electrodes of thin film transistors QA, QB, QC are connected to signal lines 34a, 34b, 34c respectively. The source electrodes of thin film transistors QA, QB, QC are respectively connected to pixel electrodes 38a, 38b, 38c of the liquid crystal cells.
To form the pixel array 30, first a patterned conductive layer, serving as scanning lines 32 and 32a, is formed on a substrate (not shown). Next, an insulating layer and a semi-conductive layer are sequentially added. Then, a second patterned conductive layer, serving as signal lines 34a, 34b, 34c, is deposited on the semi-conductive layer. Finally, a transparent conductive layer is deposited to form pixel electrodes 38a, 38b, and 38c of pixels A, B, C. An overlapping region 40a of the scanning line 32a and the pixel electrode 38a is a storage capacitor of the pixel A. Similarly, overlapping regions 40b, 40c are storage capacitors of pixels B, C. Capacitances of the storage capacitors of pixels A, B, C are represented by (CSC)A, (CSC)B, (CSC)C. The area of the overlapping region 40a is larger than that of the overlapping region 40b, whose area is larger than that of the overlapping region 40c. As a result, (CSC)A is larger than (CSC)B, which is larger than (CSC)C. Thus, feed-through voltages of pixels A, B, C, represented by (VFD)A, (VFD)B, and (VFD)C, are approximately equal (that is, (VFD)A≈(VFD)B≈(VFD)C).
In brief, the above-mentioned method adjusts the capacitances of storage capacitors to compensate feed-through voltages of all the pixels. As a storage capacitor gets farther from the scanning line control circuit, its capacitance becomes smaller. As a result, it is hard for such storage capacitor with low capacitance to help the liquid crystal cells hold electric charges. Besides, as a storage capacitor gets closer to the scanning line control circuit, its capacitance becomes larger and thus, the width of the scanning line should be made wider so as to form the storage capacitor. However, the aperture ratio of the LCD apparatus will decrease as the width of the scanning line increases.